Ion trap for cathodes



Nov. 4, 1952 J 5 5 2,617,061

ION TRAP FOR CATHODES Filed March 25, 1951 6 70 be moo 1,460 as To be mad/fled as I su77es/eoby/97S 473/0 I'NVENTOR JQHANNES DE GIER AGENT Patented Nov. 4, 1952 UNIIEDQISTATES PATENT .ION .TR'AP' FOR CATHODES Johannes de Gier, Eindhoven, Netherlands, as-

signor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application March 23, 1951, Serial No. 217,199

In the Netherlands April 12, 1950 r The invention relates to-a cathode ray tube for television reception and in particular to a cathode ray tube employing an ion trap for intercepting negative ions before striking the screen of such a tube.

An ion trap for intercepting negative ions may comprise a cylindrical electrode surrounding the electron beam which intercepts a beam of negative ions separated from the electron beam. This electrode, or ion trap, comprises a transverse wall provided with a window forthe passage of the electron beam. The cylindrical part of the ion trap may consist of a metal body arranged in the tube, which body need not necessarily have a circular cross-section. Alternatively it may consist of a conductive coating or a'metal part of the wall of the tube.

The window in the ion trap, which is provided in the transverse wall is usually given a circular shape and arranged concentrically with the axis of the ion trap, but neither the circular shape nor the concentricity are necessary.

According to the invention, the transverse wall is adapted to pass negative ions along the edge of the window for the passage of the electron beam but to a decreasing extent as the distance from the window is increased. As a result thereof more negative ions will reach thescreen than the case where the window is sharply bounded, it is true, but this apparent disadvantage is outweighed by other important advantages of the invention as will appear from the specification.

The invention will now be described in greater detail with reference to the accompanying drawing in which:

Fig. l is an axial section'of one form of ion trap for intercepting a beam of negative ions deflected from its initial direction;

Fig. 2 is an axial section of another form of ion trap, in which the electron beam is deflected from its initial direction.

Fig. 3 is a cross-section'of these members ina plane II.

Figures 4 to '8 show sever'al'embodiments of the tube according to the invention and show in different manners of perforation of the transverse wall along the edge of the window.

The ion trap shown in Fig. 1 comprises a circular-cylindrical part i and a transverse end wall 2. The end surface at the left-hand end of the cylinder I is at an angle with the axis 3 of the cylinder. At this end, where the beam 4 consisting of electrons and negative ions enters the cylinder, an electric field is produced, which has a component at right angles to the axis 3 parallel to 5 Claims. (Cl. 313- 86) the plane of the drawin in the area controlled by the beam. As a result of this field, the crosshatched ion beam 5 is'defiected in the direction of the arrow, so'that it is intercepted by, the transverse end wall 2. The axis of the deflection ion beam 5 and that of the ion trap extend in the plane of the drawing.

The deflection effect exerted by the said electric held on the electron beam is offset by magnetic forces acting at right angles to the plane of the drawing. It is immaterial whether these magnetic forces act on the electron beam over the same paths as the electric field of deflection or at other points, the result in either case being that the axis of the issuing electron beam 6 is urged .by them into the axis 3 of the cylinder.

In Fig. 2, the ion trap consists of a bent cylinder 1 having a circular cross-section, which is closed at One end by a transverse end wall 8 provided with a round window. Through this window the beam 9 consisting of electrons and negative ions enters the tube. In the bend of the cylinder a magnetic field is operative, the lines of force of which extendperpendicular to the plane of the drawing. The electron beam I0 is deflected by this field in the direction of the arrow through an angle such that the axis of the beam finally passes through the centre of the window in the transverse end wall II and is directed at right angles to the plane ofthe transverse end wall. I

The deflection effect exerted by the magnetic field on the beam of negative ions is small only so that beyond the bend of the cylinder the ions continue to move in substantially the same direction. Hence, the cross-hatched ion beam is intercepted by the inner wall of the cylinder.

In Fig. 3,'the .wall has a circular window, the radius of the circuit determining the diameter of the electron beam passing therethrough. It must 'nothav'e such a value. that the window extends into the area where the ions have a considerable density. However, this area is not sharply bound-- ed, since spreading of the ions takes place, so that the space in which ions occur is by no means limited to the cross-hatched area indicated by the outlines in Figures 1 and 2. In the direction of deflection, viewed from the axis of the electron beam, the ion density increases gradually.

The ion spot is not caused exclusively by the ions of the combined electron and ion beam entering at the end of the cylinder. Additional negative ions are produced in the cylinder, since not all the gas molecules can be removed in exhausting the discharge tube. A part of these ions move in the direction of the screen under the action of the electrostatic fields penetratin into the cylinder. Despite their low velocity they are only slightly afiected by the deflection fields. A considerable part of these ions is intercepted by the end Wall but the remainder passes through the window therein and reaches the screen. In consequence, the production of an ion spot cannot entirely be avoided by the aforesaid de-ionisation device.

The invention provides a means for reducing the troublesome influence exerted by the escaping ions on the image produced by the electrons on the screen so that the discoloration of the screen caused by the ions is less pronounced,

Fig. 4 illustrates one means for accomplishing this purpose. The wall I2 shown therein, which may be used both in the ion trap shown in Fig. 1 and in that shown in Fig. 2 and is denoted by 2 and II respectively has a substantially circular window l3 with a milled edge. The percentageof ions reaching the image screen is thus slightly increased, it is true, but since the intensity steadily decreases outwardly from the center, the ion spot is not sharply bounded at its edge but blurred, so that it is far less conspicuous. The disturbing influence on the image produced disappears for the greater part, since the transition between the part of the area of image, where the influence of the ion spot is a maximum and the part which is not reached by the ions varies gradually.

It is not necessary for the window to be milled throughout its edge. The influence of the outline is a maximum towards the side where the ion beam reaches the wall. As has been pointed out, the density of the ion current increases in this direction. Fig. 5 shows a construction in which the window is milled only at one side.

Figures 6 and 7 show two further examples of a satisfactory milling for this purpose.

Instead of providing incisions at the edge of the window, part of the wall may be made porous by providing small apertures therein. In this event the total pervious surface area per unit surface of the wall, with an increase in distance from the edge of the window should decrease to a larger extent than the corresponding density of the ion beam increases. An example of such a construction is shown in Fig. 8.

While the invention has thus been described in specific embodiments thereof, other forms and embodiments thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as defined in the claims.

What I claim is:

1. An ion trap for a cathode ray tube comprising a tubular cylindrical electrode, a transverse wall element with said electrode, said transverse wall element having an aperture therein for the passage of the electron beam, a portion of said transverse wall element surrounding said aperture bein pervious to negative ions to a degree inversely dependent on the distance from the aperture.

2. An ion trap for a cathode ray tube comprising a tubular cylindrical electrode, a transverse wall element Within said electrode, said transverse wall element having an aperture therein for the passage of the electron beam, a portion of the edge of said transverse wall element surrounding said aperture being serrated thereby providing a portion of the wall surrounding the aperture which is pervious to negative ions to a degree inversely dependent on the distance from the aperture.

3. An ion trap for a cathode ray tube comprising a tubular cylindrical electrode, a transverse wall element within said electrode, said transverse wall element having an aperture therein for the passage of the electron beam, a portion of the edge of said transverse wall element surrounding the aperture being provided with radial incisions thereby providing a portion of the surface of said wall surrounding said aperture which is pervious to negative ions to a degree inversely dependent on the distance from the aperture.

4. An ion trap for a cathode ray tube comprising a tubular cylindrical electrode, a transverse wall element within said electrode, said transverse wall element having an aperture therein for the passage of the electron beam, a portion of the edge of said transverse wall element surrounding the aperture being provided with radial conically tapering incisions thereby providing a portion of the surface of said wall surrounding said aperture which is pervious to negative ions to a degree inversely dependent on the distance from the aperture.

5. An ion trap for a cathode ray tube comprising a tubular cylindrical electrode, a transverse wall element within said electrode, said transverse wall element having an aperture therein for the passage of the electron beam, a portion of said transverse Wall element surrounding said aperture being provided with a plurality of small apertures the total surface of which decreases to a degree inversely dependent on the distance from the aperture thereby providing a wall pervious to negative ions to a degree inversely dependent upon the distance from the aperture.

JOHANNES DE GIER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,211,614 Bowie Aug. 13, 1940 2,267,083 De Gier Dec. 23, 1941 2,302,786 McGee et al Nov. 24, 1942 2,312,723 Llewellyn Mar. 2, 1943 2,515,305 Kelar July 18, 1950 2,565,357 Donal, Jr Aug. 21, 1951 

